German Patent Application Nos. 37 14 009 and 40 371 780 describe devices that detect metallically conductive parts in a substantially non-conductive conveyor stream. These devices needed, for example, for sensing metal parts in a conveyor stream consisting mainly only of paper or recyclable plastic, the conveyor stream being fed to a shredder for reprocessing. These devices are also used in pneumatic conveyor systems for granular materials made of plastic and in the tobacco and textile industries as well as in the foodstuffs industry. If solid metal parts are contained, namely, in the conveyed material, this can lead to disruptions in the work procedure or even to the destruction of machine parts although the parts might be very small. Likewise, in textile goods, metal parts and wire residue from brushing machines are disruptive for obvious reasons. On the other hand, foil-like conductive parts, such as the covers on goods packaged in plastic containers, are also a common occurrence and are generally not to be sorted out since they either serve as packaging or their mechanical resistive force is very low such that they pose no risk to subsequent machinery.
Here, the detection signal is intended to actuate protective devices, such as optical and/or acoustic signalling means, or interrupting devices for the conveyor device, or even to divert the conveyor stream containing a disruptive part into a gathering container or the like.
The devices mostly work in a manner such that an electromagnetic alternating field is established by an alternating current generator via a transmitting coil system in a section of the conveyor stream to be monitored, the amplitude and phase variation of the alternating field triggered when a part passes being detected by a receiving coil system and a subsequent evaluation circuit in the form of a phase variation signal and an amplitude variation signal, which are used to derive the detection signal. In order to be able to detect small, solid and metallically conductive parts even more reliably, in conventional devices the fact is exploited that the influence of conductive parts increases with the frequency of the electromagnetic alternating field. The conventional systems operate for this reason at relatively high frequencies of at least 16 kHz. In most cases, the frequency is considerably higher. However, the associated increase in sensitivity means that parts that are not to be sorted out per se, such as foil-like parts, trigger detection signals and depending on the type of protective means provided for the conveyor system, either shut it down, for example, or undesirably divert the conveyor stream. Nonetheless, the detection of small, solid, metallically conductive parts still frequently entails certain difficulties since the signal amplitude caused by small parts is low with respect to that of foil-like, metallically conductive parts. The state of the art is also described in general in the publication "Detecting metal particles in nonmetallic materials" in the journal "Automation", December 1965, pp. 83-87. Such measurement techniques and devices are also used to distinguish between metal types and to detect certain metallic objects, as shown by, for example, the DE-PS 28 37 265 and the DE-PS 32 28 447. According to the DE-PS 28 37 265, in order to identify conductive objects, the effective resistance component normalized to the operating frequency is measured at several frequencies and a conclusion regarding the material type is made based upon this. However, a precise investigation based on this method results in not only a high expense, but is also relatively difficult since comparison values must be retained in a memory for each material type depending on the respective object or part that is sought after. On the other hand, the solution approach described in the DE-PS 32 28 447 uses the known material-dependent phase shift between the transmitted and the received alternating field when a disruptive part passes as a criterion for the material type, but as before, there are difficulties in reliably detecting, most particularly, small but nonetheless disruptive parts.